1. Field of the Invention
The invention relates to an inverse dispersion compensating optical fiber. More particularly, the invention relates to a large-effective-area inverse dispersion compensating optical fiber that exhibits relatively low optical loss relative to conventional inverse dispersion fiber and that is suitable for compensating dispersion in large-effective-area positive dispersion fiber.
2. Description of the Related Art
Optical fibers are thin strands of glass or plastic capable of transmitting optical signals, containing relatively large amounts of information, over long distances and with relatively low attenuation. Typically, optical fibers are made by heating and drawing a portion of an optical preform comprising a refractive core region surrounded by a protective cladding region made of glass or other suitable material. Optical fibers drawn from the preform typically are protected further by one or more coatings applied to the cladding region.
Advances in transmission over optical fibers have enabled optical fibers to have enormous bandwidth capabilities. Such bandwidth enables thousands of telephone conversations and hundreds of television channels to be transmitted simultaneously over a hair-thin fiber. Transmission capacity over an optical fiber is increased in wavelength division multiplexing (WDM) systems wherein several channels are multiplexed onto a single fiber, with each channel operating at a different wavelength. However, in WDM systems, nonlinear interactions between channels occur, such as 4-photon mixing, which severely reduces system capacity. This problem has largely been solved by U.S. Pat. No. 5,327,516 (the '516 patent), which is owned by the assignee of the present application. The '516 patent discloses an optical fiber that reduces these nonlinear interactions by introducing a small amount of chromatic dispersion at the operating wavelengths. As the number of WDM channels to be transmitted over a single fiber increases, the optical power carried by the optical fiber also increases. As the optical power increases, the nonlinear effects caused by interaction between the channels also increases. Therefore, it is desirable for an optical fiber to provide a small amount of chromatic dispersion to each of the WDM channels in order to reduce the nonlinear interactions between the channels, especially in view of ever-increasing bandwidth demands. However, in order to be able to restore the signal after the transmission link, it is important that the dispersion introduced vary as little as possible amongst the different WDM channels.
Important advances have been made in the quality of the material used in making optical fibers. In 1970, an acceptable loss for glass fiber was in the range of 20 dB/km, whereas today losses are generally less than about 0.25 dB/km. The theoretical minimum loss for silica based fiber is about 0.15 dB/km, and it occurs at a wavelength of about 1550 nanometers (nm). Dispersion in a glass fiber causes pulse spreading for pulses that include a range of wavelengths, due to the fact that the speed of light in a glass fiber is a function of the transmission wavelength of the light. Pulse broadening is a function of the fiber dispersion, the fiber length and the spectral width of the light source. Dispersion for individual fibers is generally illustrated using a graph (not shown) having dispersion on the vertical axis (in units of picoseconds (ps) per nanometer (nm), or ps/nm) or ps/nm-km (kilometer) and wavelength on the horizontal axis. There can be both positive and negative dispersion, so the vertical axis may range from, for example, −250 to +25 ps/nm km. The wavelength on the horizontal axis at which the dispersion equals zero corresponds to the highest bandwidth for the fiber. However, this wavelength typically does not coincide with the wavelength at which the fiber transmits light with minimum attenuation.
For example, typical single mode fibers generally transmit best (i.e., with minimum attenuation) at about 1550 nm, whereas dispersion for the same fiber would be approximately zero at 1310 nm. Also, the aforementioned theoretical minimum loss for glass fiber occurs at the transmission wavelength of about 1550 nm. Because minimum attenuation is prioritized over zero dispersion, the wavelength normally used to transmit over such fibers is typically 1550 nm. Also, Erbium-doped amplifiers, which currently are the most commonly used optical amplifiers for amplifying optical signals carried on a fiber, operate in 1530 to 1565 nm range. Because dispersion for such a fiber normally will be closest to zero at a wavelength of 1310 nm rather than at the optimum transmission wavelength of 1550 nm, attempts are constantly being made to improve dispersion compensation over the transmission path in order to provide best overall system performance (i.e., low optical loss and low dispersion).
In order to improve dispersion compensation at the transmission wavelength of 1550 nm, it is known to couple the transmission fiber, which normally is a positive dispersion fiber (PDF), with an inverse dispersion fiber (IDF). The positive dispersion transmission fiber typically comprises a single mode fiber designed to introduce dispersion in order to reduce the nonlinear interactions between wavelength channels. The inverse dispersion fiber has a negative dispersion and negative dispersion slope that provide dispersion and dispersion slope compensation that enable the dispersion and dispersion slope compensation of the transmission fiber to be managed. The transmission PDF is coupled to a length of IDF by splicing. The combination of the PDF and the IDF has both an intrinsic fiber loss and a splicing loss. Of course, overall optical loss for a transmission link should be kept at a minimum. The need to minimize optical loss is even more important when long transmission links are involved due to the fact that more amplifiers normally are needed along the link to prevent transmission quality degeneration.
For example, in trans-oceanic communications systems it is advantageous to use a combination of super-large-effective-area (SLA) PDF and an IDF having matching relative dispersion slopes (RDSs) at a particular wavelength, usually at the center of the transmission band. The RDS of a fiber is the ratio of the dispersion slope, S, of the fiber to the dispersion, D, of the fiber. The RDS of the IDF needs to match the RDS of the PDF for proper management of dispersion and dispersion slope. However, merely matching the RDSs of the transmission PDF and the compensating IDF does not solve all problems. Other issues such as management of the aforementioned nonlinear effects, bending loss and optical attenuation should also be taken into consideration. Conventional IDF used for compensating dispersion in a SLA transmission fiber has a median loss of, for example, approximately 0.246 db/km at 1550 nm. One way to decrease the overall loss of the transmission link would be to provide an IDF that has a lower fiber loss than conventional IDF that is currently used in combination with SLA transmission fibers. However, conventional IDFs currently used with these SLA PDFs have relatively small effective areas, which presents problems. For example, the small effective area of the IDF limits the amount by which the overall nonlinear effects between channels and attenuation loss can be reduced, which limits the degree by which degradations in system transmission performance can be prevented. Of course, when system transmission performance degrades, the number of WDM channels that these systems can support becomes limited.
U.S. Pat. No. 6,301,419 B1 to Tsukitani, et al. discloses a dispersion-equalizing fiber that is designed to have reduced bending loss so that it is suitable for use in a dispersion compensating module in which it will be wound about a spool and spliced with a transmission fiber to reduce dispersion and the dispersion slope of the transmission line as a whole. Tsukitani discloses that the dispersion-equalizing fiber has an effective area of anywhere from 15 to 19 micrometers squared (μm2), and discloses that the dispersion-equalizing fiber having these effective areas restrains the aforementioned nonlinear effects. As a consequence, the dispersion-equalizing fiber has a bending loss of 10 to 50 dB/m with respect to light having a wavelength of 1550 nm when wound at a diameter of 20 mm.
Tsukitani discloses that it is desirable to maintain the ratio of the length of the dispersion-equalizing fiber to the length of the total transmission line (i.e., length of the dispersion-equalizing fiber+length of the transmission fiber), which is referred to in Tukitani as the DEF ratio, between 25% and 40% in order to repress nonlinear effects. As shown in FIG. 2B in Tsukitani, when the DEF ratio is between 25% and 40%, the effective area, Aeff, of the dispersion-equalizing fiber ranges from between about 15 and about 19 μm2. Tsukitani discloses that the nonlinearity index can be kept within acceptable ranges that provide low bending loss when the effective area of the dispersion-equalizing fiber is anywhere from about 15 μm2 to about 19 μm2 and when the ratio, Ra, between the diameter of the core region and the diameter of the trench region is greater than around 0.6. Thus, not only is the DEF ratio a factor taken into account in designing a dispersion-equalizing fiber with low bending loss, the ratio Ra is also taken into account. The diameter of the core region generally doesn't change very much compared to the amount by which the diameter of the trench region may vary. It can be seen from FIGS. 3-9 in Tsukitani that when the effective area Aeff is anywhere from about 15 μm2 to about 19 μm2 and Ra is greater than 0.6, the dispersion-equalization fiber exhibits a relatively low 20 mm bending loss and has a good nonlinear index.
One of the disadvantages of the dispersion-equalization fiber disclosed in Tsukitani is that, as can be seen from the figures in Tsukitani, increases in the effective area Aeff and/or R4 reduce the ability of the fiber to repress nonlinear effects and/or increase bending loss. It would be desirable to provide an inverse dispersion fiber (IDF) that has a large effective area Aeff and that is capable of maintaining the aforementioned desirable transmission characteristics, such as, for example, low attenuation loss, reduced nonlinear interactions between channels, etc, even with an Ra less than or equal to, for example, 0.45. It would also be desirable to provide a large-effective-area IDF that has a low cable cutoff wavelength (e.g., below 1500 nm) and bending loss sensitivities that will result in a reduction in cabling optical loss.